It is known that a latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are known for forming latent electrostatic images. For example, one method is providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. Useful liquid toners comprise a thermoplastic resin and dispersant nonpolar liquid. Generally a suitable colorant is present such as a dye or pigment. The colored toner particles are dispersed in the nonpolar liquid which generally has a high-volume resistivity in excess of 10.sup.9 ohm centimeters, a low dielectric constant below 3.0 and a high vapor pressure. The toner particles are less than 10 .mu.m average by area size. After the latent electrostatic image has been formed, the image is developed by the colored toner particles dispersed in said dispersant nonpolar liquid and the image may subsequently be transferred to a carrier sheet.
Since the formation of proper images depends on the differences of the charge between the liquid developer and the latent electrostatic image to be developed, it has been found desirable to add a charge director compound of the ionic, nonionic, or zwitterionic type and preferably a charge adjuvant such as a polyhydroxy compound, an aminoalcohol, polybutylene succinimide, aromatic hydrocarbon, metallic soap, etc. to the liquid toner comprising a thermoplastic resin, dispersant nonpolar liquid and preferably a colorant. Such liquid toners, while developing good quality images having high resolution with reduced wicking and squash, still do not provide the quality images required for certain end uses, e.g., optimum machine performance in digital color proofing. It has been found that some developer formulations suffer one or more of the following disadvantages which affect image quality: the image formed shows beading or flow and toner particle size is too large.
It has been found that the above disadvantages can be overcome and an improved electrostatic liquid developer prepared containing a nonpolar liquid, a thermoplastic resin, a nonpolar liquid soluble ionic, nonionic or zwitterionic compound, optionally a colorant, and the adjuvant compound of this invention. Such electrostatic liquid developer when used to develop an electrostatic image results in improved image quality, and solid area coverage independent of the charge director compound present.